Q.* now forms its products in the order a1*b1 (low cell), a0*b1, a1*b0,
a0*b0, dropping each input after its last use, with b0 waiting on the
return stack. Each sign correction, (b1<0 ? a0 : 0) and (a1<0 ? b0 : 0),
is folded into cell 2 as soon as its operands are adjacent, using -if
rather than 0< calls. At most four live values sit under any UM* call.
Headroom (data cells under args / return entries): 2/2 -> 3/3.
Same checks as before at 32- and 64-bit cells, optimised and ASan+UBSan;
dropping either sign correction fails about 9900 checks.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
